1. Field of the Invention
The subject invention generally pertains to pliable air ducts and more specifically to the air permeability of such a duct.
2. Description of Related Art
In HVAC systems (heating, ventilating, air conditioning), conditioned supply air discharged from a blower is often conveyed to various rooms or areas within a building by way of ductwork. Conventional ductwork is typically formed of sheet metal and is often installed above ceilings for convenience and aesthetics. But in warehouses, manufacturing plants and many other buildings, the ducts are suspended from the roof of the building and are thus exposed. This not only creates a poor appearance in many cases, but can create other problems as well.
For example, temperature differentials between an air duct and the air on either side of the duct wall can create condensation on both the interior and exterior of the duct. The presence of condensed moisture on the interior of the duct may form mold or bacteria that the duct then passes onto the room or other areas being supplied with the conditioned air. If an exposed sheet metal duct conveys relatively cool air, condensation can form on the exterior of the duct. The condensate may then drip onto the floor, inventory, and personnel below. The consequences of the dripping can range anywhere from a minor irritation to a dangerously slippery floor for the personnel, or complete destruction of the products it may drip on (especially in food-processing facilities).
Further, metal ducts with localized discharge registers have been known to create uncomfortable drafts and unbalanced localized heating or cooling within the building. In many food-processing facilities where the target temperature is 42 degrees Fahrenheit, a cold draft can be especially uncomfortable and perhaps unhealthy.
Many of the above problems associated with exposed metal ducts are overcome by the use of fabric ducts, such as DUCTSOX fabric ducts by Frommelt Safety Products Corporation of Milwaukee, Wis. Such ducts typically have a fabric wall that is air-permeable to broadly and evenly disperse the air into the room being conditioned or ventilated. If greater airflow in needed in certain areas, the fabric duct can be provided with additional discharge openings, such as air registers or cutouts in the fabric.
The porosity of conventional fabric can pass a substantial amount of air, which can be desirable in many applications where the airflow through the pores of the fabric is used primarily for evenly dispersing air into a room. However, some applications require airflow that is more directed toward certain areas of a room. In such cases, it may be desirable to have relatively large discharge openings provide most of the air airflow, while the pores of the fabric provide only enough airflow to inhibit dust and condensation from accumulating on the outer surface of the fabric material.
Unfortunately, it can be difficult to acquire an air duct material whose porosity provides an appropriately small amount of airflow, such as 2 cfm (two cubic feet per minute of air across one square-foot of material subject to a 0.02 psi air pressure differential). Standard fabric materials have been found to pass 40 cfm or more. Such materials have been calendered in an attempt to reduce the materials porosity. Although calendering conventional fabric does reduce its porosity temporarily, much of the effect is lost after the material is washed. Thus, simply calendering just any porous fabric is not a permanent solution to the problem.